A variety of methods are known for determining or estimating the stroke volume, i.e., the pumping capacity of the heart. Dilution methods (primarily thermal ones) are most common. Thermal dilution methods are disadvantageous for use in implants owing to their high power consumption and the need to provide access from outside of the body for delivering a temperature pulse. Additionally, continuous measurements using dilution methods are subject to significant limitations. Methods based on measuring the blood flow are also known, but the arrays used for measuring the blood flow using implanted sensors are relatively technically complex, and/or have high power requirements to measure or process the sensor signals.
Further methods employ intracardiac or intrathoracic impedance signals for deriving parameters used as a measure of the stroke volume or the change thereof (as seen, for example, in U.S. Pat. No. 7,395,114, EP 1 762 270, and U.S. Pat. No. 7,702,389). In the impedance-based methods, the parameters are determined from conductivity changes, which are only indirectly associated with the stroke volume and which are influenced by other factors. For example, U.S. Pat. No. 7,395,114 derives a degree of the ventricular contraction from the intracardiac impedance, and uses it to draw a conclusion regarding the stroke volume.
Measures of stroke volumes can also be determined using echocardiography. For example, geometric changes of the left ventricle can be determined and used to derive a measure of the stroke volume. Furthermore, the blood flow in the aortic arch can be approximately measured using the Doppler effect and used to estimate the stroke volume. However, these methods are undesirably complex for use in implants.
Methods have also been developed wherein parameters associated with measured cardiac sounds, under certain assumptions, correlate with the stroke volume. Frequently, acceleration sensors are employed for recording the cardiac sounds.
In U.S. Pat. No. 7,139,609, the cardiac sounds are isolated from the signal of an acceleration sensor, such as by suitable filtration. Based on empirical relationships, these cardiac sounds, primarily the first heart sound S1, are used to derive parameters which, under certain conditions, are related to the stroke volume. The cardiac sounds are additionally analyzed is with respect to characteristics that indicate impaired heart valve function in order to perform limited corrections to the calculated parameter, if necessary. Furthermore, the intracardiac electrogram (IEGM) is used in the determination of the parameters, requiring an additional electrode. Since a variety of other factors influence the characteristics of the cardiac sounds, it is apparent that the determination of the parameters from the cardiac sounds and the relationship thereof with the stroke volume is not very reliable. A similar method is disclosed, for example, in U.S. Pat. No. 7,585,279, in which a measure of the stroke volume is likewise approximated based on the heart sound S1.
It is known that other components can also be isolated from the signals of an acceleration sensor by suitable signal processing. One of these components is referred to as the seismocardiogram (SCG), which represents the vibration signals created by the beating of the heart in the thorax. The SCG contains the acceleration signals created by the mechanical movement of the heart in the thorax perpendicular to the body axis. This too can be used to determine parameters which, under certain conditions, are approximately related to the stroke volume, for example, according to U.S. Pat. No. 6,978,184. However, this process assumes a defined and fixed relationship between the mechanical movement of the heart and the pumping capacity, which does not unconditionally apply, especially in the case of cardiac insufficiency.